The invention relates to the filtration of gases, typically of air to remove dust particles. The particles may range from about 0.3 to about 30 micron in size.
It is known to make a gas filter bag from gas permeable natural material in the form of generally parallel pockets. The pockets have an open upstream inlet and a closed bottom end, the intention being that the dust particles will be trapped in the closed end of the pockets as the gas flow passes therethrough. The pockets may be defined by longitudinal seams formed by stitching. Usually a hot melt adhesive is applied over the stitched holes to seal them to avoid escape of particles. This is a labour intensive manufacturing method and the gas entrapment properties are not very good. It is one object of the invention to provide a gas filter of improved performance.
According to the invention in one aspect there is provided a gas filter element comprising
two side walls forms of gas permeable material joined along three of their sides to define a pocket having an upstream open inlet and a downstream closed end, characterised in that generally parallel partitions bridge the opposite side walls to define individual pocket portions extending from the inlet end to the closed end, marginal portions of the partitions being secured to the inside of the side walls, whereby in use the air filter element exhibits a low pressure drop and a high degree of dust efficiency and dust holding capacity.
The walls may be made of any suitable material which may be wholly or mainly synthetic fibres. Because the partitions are joined to the walls by a welding process, the material should be fusible (or at least thereof should be).
Preferably the partition is a thin generally planar strip of fusible material which in use presents a narrow edge to the flow of air.
Preferably the marginal portions are secured to the inner surface of the side walls by ultrasonic welding.
Preferably a short length portion of one edge of the partition adjacent the open inlet end is not joined to the inner surface of the adjacent wall to permit flexure of the front end of the pocket so that it may easily be mounted in say a filter header frame. Alternatively and more preferably an elongate slit is formed in the upstream end portion of the partition to provide two flaps which enable the pocket to be opened in the manner of a jaw to engage the frame. The length and shape of the slit will be determined according to each filter element and frame, which typically is a header frame.
In another aspect the invention provides a continuous method of making a succession of pockets for a gas filter element, the method comprising:
advancing two elongate webs of synthetic fibre material to a work station to lie on opposite sides of an ultrasonic welding unit; PA1 advancing parallel strips of fusible material to extend generally parallel to each other and to be positioned between the webs and substantially at right angles to the major axis of the webs; PA1 passing the strips through a former to fold up the longitudinal marginal portions so that the marginal portions lie on the inner surface of the respective webs; PA1 passing the webs and the marginal portions through the ultrasonic welding unit so as to ultrasonically weld the marginal portions to the respective web and so connect each side of the strip to the webs to form bridging portions in between the webs; PA1 sealing the side portions of the webs; PA1 cutting across the sealing the side sealed webs to form a closed end of the gas filter element and separate it from the webs.